henry copeland:

Opioids produce analgesia via immune cells: Researchers from the Charité find new signaling pathway -- ScienceDaily

"We were able to show that opioid agonists activate opioid receptors on immune cells, which triggered the release of endogenous painkillers (opioid peptides) and produced analgesia in a mouse model of neuropathic pain," explains Prof. Machelska. She adds, "This led us to conclude that opioids can exert enhanced analgesia when they act directly in painful tissue -- providing that this tissue is inflamed and contain immune cells."
- www.sciencedaily.com